They are extremely fine. Forget the Premier, this is the gun.
The breech seal is meant to go in counterintuitively the square face inward and the slight radius facing the barrel.
The phosphor bronze piston ring can last forever. Simply lube with 2 stroke motor oil.
I have had Seniors running 500 fps but no other webley over lever pistol gets as close…its to do with the volume the piston ring allows.
The disadvantage of the Senior is that ring needs constant lubrication but you can do that.
If you need any advice just ask….ive had many and tuned too many to remember.